The BTB domain of the fruitless gene in Drosophila : a comparative study

The BTB domain of the fruitless gene of each of the eight species of the melanogaster species subgroup was cloned and sequenced. The intron-exon organization among the eight species is highly conserved. All eight species contain two closely spaced introns at the exact same location within the coding sequences of the BTB domain. The size of the first intron varies among the eight species from 63 to 73bp, while the size of the second intron ranges from 282 to 320bp. The coding sequences of the BTB domain are nearly exactly conserved, with only a few sites of silent substitution. On the other hand, the two introns show a wide range of sequence variation. Surprisingly, the smaller intron in the Muscle of Lawrence positive (MOL+) species is highly conserved both in size and in DNA sequence, and contains two putative transcription factor binding sites for the Deformed and Bicoid proteins, important in embryonic development. The predicted Bed binding site is substantially altered in all the MOL- species. Thus, these comparative sequence data lead to two intriguing molecular models. (1) Modulatory sites of gene expression may lie within gene introns. (2) The evolution of such an enhancer sequence may correlate with the evolutionary path of an anatomical structure. In particular, alteration of the Bed binding site could be a factor in the evolutionary loss of the MOL in many species. By comparison, the larger intron shows more sequence divergence, but with two highly conserved blocks near the 3' splice site. One of the blocks contains sequence almost identical to the consensus 13- nucleotide Transformer protein binding site. RT-PCR of the BTB-domain region suggests alternative splicing patterns in the adult. A threshold theory which explains the different MOL expressions is presented.